Fish skeletons with hyperostosis in the Ichthyological Collection of the Natural History Museum Vienna (Austria)

Fish skeletons with hyperostosis in the Ichthyological Collection of the Natural History Museum Vienna (Austria) Harald Ahnelt1,2, Julia Sara Klein2, Viola Winkler3 1 Natural History Museum Vienna, First Zoological Department, Burgring 7, 1010 Vienna, Austria 2 University of Vienna, Department of Evolutionary Biology, Djerassiplatz 1, 1030 Vienna, Austria 3 Natural History Museum Vienna, Central Research Laboratories, 3D-Laboratory, Burgring 7, 1010 Vienna, Austria https://zoobank.org/671CF6A0-3AFD-4E73-A509-5963E43F639A Corresponding author: Harald Ahnelt ([email protected], [email protected]) Academic editor: Nesrine Akkari ♦ Received 28 September 2024 ♦ Accepted 27 August 2025 ♦ Published 10 November 2025 Abstract The term “hyperostosis” refers to an overgrowth of bone, which results in an increase in bone volume and/or compactness localized in vertebrate skeletons from fish to mammals including men. Hyperostosis can originate pathologically but also non-pathologically. In latter case, it is often species specific. Such species specific hyperostoses are known especially from marine acanthopterygian fishes with anosteocytic (acellular) bone. These hyperostotic bones are characterized by an increase of their volume giving them a globular, swollen appearance. Generally, the hyperostotic bones of Acanthopterygii have a spongy structure but there are also few cases where they are massive and compact. The Ichthyological collection of the Natural History Museum Vienna (NHMW) houses a collection of skeletons from species which exhibit a “typical” hyperostosis (voluminous bones of spongy structure) but also that characterized by increased volume of bones with compact structure. Furthermore, based on skeletons of juvenile specimens, we can demonstrate that the timing of onset of hyperostosis is not simultaneous but sequential. Key Words Acanthopterygii, bone, fish hyperostosis, Os wormianum Introduction The term “hyperostosis” refers to an overgrowth of bone, a hyperossification, which results in an increase in bone volume and/or compactness localized in vertebrate skeletons from fish to mammals, including men (Köstler 1882; Abel 1912; Lallo et al. 1977; Smith-Vaniz et al. 1995; dos Reis et al. 2020). Various terms have been applied to the phenomenon of bone hypertrophy in the past, e.g., exostosis (Schlumberger and Lucké 1948; Chiodini and Nielsen 1983), hyperostosis (Meunier 2008; Jawad 2013), fish hyperostosis (Glowacki and Kaufmann 1992; Smith-Vaniz et al. 1995) or pachyostosis (Abel 1912; Houssaye 2009). Because hypertrophic bone differs from normal bone by high radiographic densities (Aguilera et al. 2017), such dense bony structures were sometimes erroneously also interpreted as an osteoma (Schlumberger and Lucké 1948; Stolk 1958; Capasso 1997). Hyperostotic or “swollen” bones are since long known from extant (Bell 1793; Köstler 1882) and fossil (Steindachner 1859; Beneden 1881) teleost fishes. The first descriptions of hyperostotic bones date back to the 17th century (Worm 1655; Olearius 1674; Jacobaeus 1696). This phenomenon “hyperostosis” is applied to skeletal thickening of a variety of mostly tropical and subtropical marine fishes of taxonomical diverse groups and was termed fish hyperostosis (Glowacki and Kaufman 1992; Smith-Vaniz et al. 1995). An overgrowth of bone can originate pathologically (Walker et al. 2009; Fjelldal et al. 2018) but also naturally and in latter cases is often species specific (Rao and Lakshmi 1986; Smith-Vaniz et al. 1995; Meunier et al. 2010; Starks 2011; Aguilera et al. 2017). Such species specific hyperostosis has been found especially in fishes ANHMW 126 2025, 5–25 DOI 10.3897/anhmw.170482 Copyright Harald Ahnelt et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. anhmw.pensoft.net Harald Ahnelt et al.: Fish skeletons with hyperostosis in NHMW6 (Smith-Vaniz et al. 1995; Starks 2011; Tuna 2015; Aguilera et al. 2017), but also in some amphibians (Clemente-Carvalho et al. 2009; dos Reis et al. 2020). Hyperostotic bones were predominantly documented in Acanthopterygii, the most species rich and evolutionary most advanced group of teleost fishes (Smith-Vaniz et al. 1995). Acanthopterygii in particular and Neoteleostei in general, are synapomorphic in the type of their skeleton which is, contrary to non-neoteleosts, formed by acellular (anosteocytic) bone (Kölliker 1859; Korschelt 1938; Ekanayake and Hall 1987; Davesne et al. 2019). For exceptions and a detailed review of the origin and evolution of acellular bone in teleost fishes we refer to Davesne et al. (2019). Generally, hyperostotic bones are characterized by an increase of their volume giving them a globular, swollen appearance (Korschelt 1938; Driesch 1994; Smith-Vaniz et al. 1995). This thickness is the result of two processes: (1) excessive periosteal osteogenesis followed by (2) resorption which results in the spongy structure of these bones around the pre-existing acellular bone (Glowacki and Kaufmann 1992; Smith-Vaniz et al. 1995). Generally, the spongy structure is surrounded by a thin sheath of compact bone (Meunier et al. 2010). Fish hyperostosis affects bones from different regions of the skeleton like the bones of the skull, the pectoral girdle or the vertebral column (summarized in Smith-Vaniz et al. 1995, 2024). It is believed that, at least in some species, such hyperossified bones occur (1) in a species specific pattern (Korschelt 1938, 1940; Smith-Vaniz et al. 1995), (2) symmetrically in the skeleton as long as paired bones are affected (Korschelt 1938, 1940) and (3) only in large, adult specimens (Bell 1793; Korschelt 1938; Smith-Vaniz et al. 1995). For a detailed review of hyperostosis in acanthopterygian fishes we refer to Smith-Vaniz et al. (2024). Here, we describe the hyperostotic fish skeletons of the Ichthyological Collection of the Natural History Museum Vienna. These skeletons exhibit two different types of hyperostosis, (1) “typical” hyperostosis with voluminous bones of spongy structure and (2) bones with increased volume but with compact structure. Materials and methods The collection of hyperostotic fish skeletons in the Naturhistorisches Museum Wien (NHMW) comprises 12 specimens of seven species belonging to six families and four orders. The classification follows Eschmeyer’s Catalog of Fishes (Fricke et al. 2025). Figure 1. Franz Steindachner when he was Intendant (Director) of the k.k. naturhistorisches Hof-museum surrounded by various objects. To the right a hyperostotic fish skeleton (Chaetodipterus faber) is pictured. Painting Josef Engelhart, 1911. Photo by H. Ahnelt. Annals of the Natural History Museum Vienna 126 2025, 5–25 anhmw.pensoft.net 7 Studied material Acanthuriformes Jordan, 1923 Family Drepaneidae Gill, 1872 Drepane punctata (Linnaeus, 1758) Two specimens; one adult specimen, whole mount NMW 93893, 280 mm SL, Dakar, Senegal; one juvenile specimen, whole mount NMW 60004, 145 mm SL, Java, Indonesia. Family Ephippiidae Bleeker, 1859 Chaetodipterus faber (Broussonet, 1782) One specimen, whole mount NMW 93686, 165 mm SL, Paramaribo, Suriname. Family Sciaenidae Cuvier, 1829 Paralonchurus peruanus (Steindachner, 1875) One specimen, whole mount NMW 93753, 350 mm SL, Callaõ, Peru. Carangiformes Jordan, 1923 Family Carangidae Rafinesque, 1815 Caranx hippos (Linnaeus, 1766) One specimen, whole mount, NMW 93700, 685 mm SL, Rio de Janeiro, Brazil. Caranx fischeri Smith-Vaniz & Carpenter, 2007 One specimen, whole mount, NMW 60024, 373 mm SL, Dakar, Senegal. [The specimen is displayed in the public exhibition of the museum as Caranx hippos]. Scombriformes Woodward, 1901 Family Trichiuridae Rafinesque, 1810 Lepidopus caudatus Euphrasen, 1788 One specimen, whole mount NMW 93314, 1342 mm SL, Tenerife, Canary Islands, Spain. Zeiformes Berg, 1937 Family Zeidae Rafinesque, 1815 Zeus faber Linnaeus, 1758 Five specimens; NMW 60029, 350 mm SL, whole mount, Europe (no detailed locality is known); NMW 93930, 159 mm SL, whole mount, Europe (no detailed locality is known); NMW 60021, postcranial skeleton of 324 mm length including caudal fin, head and girdles are missing, whole mount, Trieste, Italy; NMW 92270, cranial skeleton, branchial basket, shoulder girdle, pelvic girdle, single bones mounted on plate, no detailed locality is known; NMW 91917, cranial skeleton (in part), branchial basket, shoulder girdle, pelvic girdle, single bones mounted on plate, Trieste, Italy. All these skeletons can be traced back to the rich scientific life of Franz Steindachner, Curator of the Ichthyological and Herpetological Collections and Head (Intendant) of the k.k. naturhistorisches Hof-Museum (Fig. 1), the forerunner of the Naturhistorisches Museum Wien (Natural History Museum Vienna). Additional comparative material Two anal-fin pterygiophores, “Ossa wormiana”, NMW 97939. Zeus faber, two specimens, whole mount skeleton without hyperostosis, NMW 60066, 347 mm SL, Atlantic (exact locality is not known); alcohol preserved specimen, NMW 12759, 324 mm SL, Trieste, Italy. Paralonchurus peruanus, x-rays of three syntypes, NMW 15812, 150 mm SL, NMW 78571, 230 mm SL, NMW 84795, 320 mm SL, Callaõ, Peru. Drepane longimana (Bloch & Schneider, 1801), x-ray, NMW 90900, 150 mm SL. Methods Radiographs and photographs Images of skeletons and bones were taken from the left side in lateral view. Otherwise, this will be noted in the corresponding figure caption. MicroCT imaging For the analysis of the internal morphology of the hyperostosis individual bones of the specimens listed below were imaged in NHMW using micro-computed tomography (microCT, YXLON FF35 CT equipped with a FXE Transmission Beam and Y.Panel 4343 CT Csj flat panel detector). All samples were scanned with an energy of 70 kV and electric current of 450 µA. Except for one bone (NMW 60029) a helical acquisition was used and several thousand projection images taken, that were then reconstructed with voxel sizes ranging from 10.10 µm to 19.1 µm. The image stacks were visualized in Dragonfly, Version 2020.2 for Windows (Comet Technologies Canada Inc., Montreal, Canada). One anal-fin pterygiophore, Os wormianum, NMW 979394. Zeus faber, NMW 92285, dorsal-fin spine; NMW 60029, buckler scale; NMW 91917, supracleithrum. Caranx hippos, NMW 93700, rib. anhmw.pensoft.net Harald Ahnelt et al.: Fish skeletons with hyperostosis in NHMW8 Identification of the skeleton of Caranx fischeri (NMW 60024) based on hyperostosis Hyperostosis occurs in Carangidae in a species-specific pattern, which enables to identify species on the basis of their hyperostosis (Smith-Vaniz and Walsh 2019). The specimen was labeled as Caranx hippos, but although hyperostosis is well documented from many specimens of this species (Smith-Vaniz et al. 2024, this study), the posttemporal was never affected. However, there are three species of Carangidae from which hyperostotic posttemporal bones have been described: Caranx caninus Günther 1867, Caranx crysos (Mitchill 1815) and Caranx fischeri. The first species, Caranx caninus, is a Pacific species but the specimen under investigation was collected at the east African coast off Dakar (Senegal). And contrary to Caranx fischeri, the other species also occurring in the East Atlantic, Caranx crysos, has no hyperostotic ribs. Additionally, the lobes of the second dorsal fin and of the anal fin are prominent vs. moderately enlarged in Caranx crysos, and the number of segmented anal-fin rays is 17 and lower than in Caranx crysos (anal-fin rays 19–21) (Smith-Vaniz and Walsh 2019). Abbreviations and acronyms NHMW, NMW Natural History Museum Vienna SL Standard length; measured from the tip of the snout to the posterior margins of hypurals. Results Os wormianum Fig. 2 The “Os wormianum” is the hyperostotic first pterygiophore of the anal fin. Worm (1655) described and illustrated such a hyperostotic fish bone, although this bone was a mystery for him. Later authors named it “Os wormianum”. There are three “Ossa wormiana” in the Ichthyological Collection (NHMW). One is complete and has a length of 102 mm, while in the two others the proximal parts are broken and missing (70.6 mm and 96.0 mm respectively). The distal part of all three “Ossa wormiana” Figure 2. “Os wormianum”. A. Drawing of the mysterious mouse-shaped bone by Ole Worm (1655); B. NMW 97939, hyperostotic first anal-fin pterygiophores (“Ossa wormiana”) in lateral view with the first two “peak-like” anal-fin spines (arrows) in situ; asterisk = base for the third anal-fin spine (this spine is missing). Scale bar: 20 mm. [See Fig. 17 for the position of the first anal-fin pterygiophore in situ.] Photo by H. Ahnelt. Annals of the Natural History Museum Vienna 126 2025, 5–25 anhmw.pensoft.net 9 is greatly gall-like expanded and roundish. On the ventral side, these pterygiophores carry the first and the second spines of the anal fin (Fig. 2). These are movable connected to the base of the pterygiophore which also has a short cylindrical base for a third spine, but this third spine is generally detached and is not present on the pterygiophore (Fig. 2). The first figure (Bell 1793, fig. 2), which shows this bone as part of a fish skeleton, for a long time considered enigmatic, is reproduced here (Fig. 3). The order of orders and families follows their phylogenetic position according to Near and Thacker (2024). Zeiformes Berg, 1937 Zeidae Rafinesque, 1815 Zeus faber Linnaeus, 1758 Figs 4–10 Bones of the cranial and the postcranial skeleton were affected by hyperostosis. Characteristic for Zeus faber is, that from the entire internal postcranial skeleton only the shoulder girdle shows hyperostotic swellings. On the other hand, some specialized scale-like structures showed a dramatic increase of size. A skeleton of nearly the same size was available which showed no signs of overgrowth of bone. Following bones were hyperostotic: on the head the posterior half of the lacrimal, the frontal in the shape of a triangular, narrow “helmet” and the ventral half of the preopercle (Figs 4, 5), from the postcranial skeleton the supracleithrum and the spur of the cleithrum (Figs 4, 5). Additionally, the spines 3–8 of the first dorsal fin were very massive and especially their bases wing-like enlarged (Figs 4, 5). At the bases of the second (soft) dorsal fin and of the anal fin extended large roundish and bulky bony scutes, the so called “bucklers” or “buckler scales”. Those of the dorsal row were distinctly larger than those of the ventral row (Figs 4, 6, 7). Contrary to fishes with the “typical” spongy hyperostosis which generally occurs in large adults, the hyperostosis in Zeus faber seemingly starts much earlier in ontogeny (Figs 8, 9). The smallest specimen available as skeleton with hyperostosis has a SL of 159 mm (Fig. 9). It shows hyperossifications on the frontal, the ventral half of the preopercle, the wing like lateral extensions Figure 3. Skeleton of the longfin batfish Platax teira. The first picture of a fish with hyperostotic bones in situ (Bell 1793). Drawing is reversed to the left. Arrow indicates the first pterygiophore of the anal fin, the “Os wormianum”. anhmw.pensoft.net Harald Ahnelt et al.: Fish skeletons with hyperostosis in NHMW10 of the first dorsal-fin spines and the bony scutes (buckler scales) at the base of the second (soft) dorsal fin. The spur of the cleithrum, distinctly enlarged in adult specimens (Fig. 5A), shows no sign of hyperostosis and also the bony scutes at the base of the anal fin are still not enlarged at this size of the specimen, but those on the base of the dorsal fin already are enlarged. Unfortunately, both supracleithra of this specimen are not preserved. Already distinctly enlarged is the spur of the cleithrum in two other subadult specimens (Fig. 8). Here, the bones of the head (skull and branchial basket) and of both girdles (pectoral and pelvic) are not in situ mounted but singly on a wooden plate. Therefore, the size of the specimen can only be estimated. Comparing with a juvenile specimen (Fig. 9) these specimens had approximately the size of 190–200 mm SL. Besides the cleithral spur also Figure 4. Zeus faber. A. NMW 60029, 350 mm SL; skeleton with hyperostotic bones; B. NMW 60066, 347 mm SL; skeleton not affected by hyperostosis. Abbreviations: BK: buckler scales on base of second dorsal and anal fin, CL: cleithrum, DS: spines of first dorsal fin, FR: frontal, LA: lacrimal (first infraorbital), POC: preoperculum, SCL: supracleithrum. Scale bars: 50 mm. Photos by H. Ahnelt. Annals of the Natural History Museum Vienna 126 2025, 5–25 anhmw.pensoft.net 11 Figure 5. Zeus faber, head in lateral view. A. NMW 60029, with hyperostotic bones (from Smith-Vaniz et al. (2024), modified); B. NMW 60066, no hyperostotic bones. Abbreviations: CL: spur of the cleithrum, DS: bases of the spines of the first dorsal fin, FR: frontal, LA: posterior half of the lacrimal (first infraorbital), POC: ventral half of preopercle, SOC: supracleithrum. Same bones are white encircled. Scale bars: 50 mm. Photos by H. Ahnelt. anhmw.pensoft.net Harald Ahnelt et al.: Fish skeletons with hyperostosis in NHMW12 the supracleithrum is distinctly hyperostotic enlarged. And the ventral part of the preopercle also shows distinct signs of hyperostosis. In adult specimens are all hyperostotic bones clearly visible on the outside which gives the fish a spiny impression (Fig. 10). Figure 6. Zeus faber postcranial skeletons. A. NMW 60021; B. Single bones mounted on plate, NMW 92285. Hyperostotic bones are dorsal spines and bony scutes (buckler scales) at the base of the second (soft) dorsal and the anal fin. In B the buckler scales are positioned dorsally and ventrally to the respective fins and the dorsal spine 5 is missing. Scale bars:100 mm. Photos by H. Ahnelt. Annals of the Natural History Museum Vienna 126 2025, 5–25 anhmw.pensoft.net 13 Scombriformes Woodward, 1901 Trichiuridae Rafinesque, 1810 Lepidopus caudatus (Euphrasen, 1788) (as Lepidopus ensiformis Vandelli, 1799) Fig. 11 Only the dorsal-fin pterygiophores are affected by hyperostosis. Ten pterygiophores were roundish inflated, i.e., the pterygiophores 3–6, 9–11, 13, 20 and 38. All these are pterygiophores of the abdominal part of the fish. No such bony increases were found on pterygiophores of the dorsal fin and of the anal fin of the tail (Fig. 11). Carangiformes Jordan, 1923 Carangidae Rafinesque, 1815 Caranx hippos (Linnaeus, 1766) Fig. 12 With a length of 83 cm, this skeleton is the most impressive of the skeletons with hyperostotic bones in the collection of the Natural History Museum of Vienna. Hyperostotic structures occurred only in the postcranial skeleton. Affected were the ribs of the vertebra 6–9, the first pterygiophore of the first dorsal fin, the 10th and the 12th pterygiophore of the second dorsal fin, the first pterygiophore of the anal fin, Figure 7. Zeus faber. Comparison of hyperostotic A. NMW 60092 and non-hyperostotic; B. NMW 60066 first dorsal-fin spines and the bony scutes along the base of the second (soft) dorsal fin. a = anterior. Scale bars: 20 mm. Photos by H. Ahnelt. anhmw.pensoft.net Harald Ahnelt et al.: Fish skeletons with hyperostosis in NHMW20 only the frontal bone shows first signs of overgrowth of bone. Supraoccipitale, and pterygiophores of the dorsal and the anal fin are not affected at this size. To be noted that the anterior part and the crest of the supraoccipital of the juvenile specimen are built by a much more massive bone than its posterior part, which is thin and translucent. This bone has entirely the same coloration than the other bones of the skull and hyperostosis is (at least nonhistologically) not apparent like it is in the frontal bone. Here, the onset of hyperostosis has obviously already started. The part of the frontal with bone overgrowth is whitish. Ephippiidae Bleeker 1859 Chaetodipterus faber (Broussonet, 1782) Fig. 19 This skeleton made it into the painting by Josef Engelhart on the side of Franz Steindachner (Fig. 1). Bones of the cranial and of the postcranial skeleton were hyperostotic. Affected were the supraoccipital of the skull, the ribs of the vertebra 3–7, the first and the second pterygiophore of the first dorsal fin, the first pterygiophore of the anal fin, Figure 18. Drepane punctata. A. NMW 93893, 280 mm SL; skeleton of an adult specimen with hyperostotic bones. Scale bar is 50 mm. B. NMW 60004, 145 mm SL; skeleton of a juvenile specimen, hyperostosis started at the frontal bone (arrow). For further explanations see text. Abbreviations: APT: first pterygiophore of the anal fin, FR: frontal, PT: first pterygiophore of the first dorsal fin, SOC: supraoccipital. Scale bars: 50 mm (A); 30 mm (B). A: Photo by K. Schiller, B: Photo by H. Ahnelt. Annals of the Natural History Museum Vienna 126 2025, 5–25 anhmw.pensoft.net 21 typically the shape of an “Os wormianum” (Fig. 2), the 2nd to the 6th haemal spine and the ventral part of the cleithrum. The supraoccipital was massive swollen and gave the impression of a helmet. The ventral part of the cleithrum was also severely hyperossified and looked like a ruff (Fig. 19). The ribs were cylindrical inflated along their entire length, narrowing in diameter from anterior to posterior. MicroCT investigation Figs 20, 21 All skeletons, except those of Zeus faber, exhibit an internal structure of their hyperostosis typical for acanthoptherygian fishes with anosteocytic (acellular) bone. The affected parts of the bones are spongy and surrounded by a thin sheet of compact bone as shown in a first anal-fin pterygiophore, likely of Chaetodipterus faber (Fig. 20A) and in a rib of Caranx hippos (Fig. 20B). Hyperostotic bones of Zeus faber, a paracanthopteygian, are characterized by compact, laminar bone, e.g., the supracleithrum (Fig. 20C), a dorsal buckler scale (Fig. 20D, E) or a dorsal fin spine (Fig. 20F, G) of NMW 60029, although this species has anosteocytic (acellular) bone (Meunier and Béarez 2019). Discussion The enigmatic Os wormianum, an anal-fin pterygiophore Worm (1655) described and illustrated a hyperostotic fish bone, although this bone was a mystery for him. He wrote: “A strange bone was handed to me that resembles a mouse in shape. It has a thick, spherical belly with two moveable and pointed beaks … followed by a long, narrow tail .... I confess that I do not know from which animal and from which part of the body this bone comes.” In honor of Ole Worm, who was the first to study this enigmatic bone, later generations named it Os wormianum (Fig. 2). It has to be noted that the term “Os wormianum” is also in use for accessory bones in the scull of humans (Shapiro and Robinson 1976; Klunker 2014). The “moveable and pointed peaks” mentioned by Worm (1655), are actually the first and the second anal-fin spines, which are moveable articulated to the base of the pterygiophore. It took more than 100 years before the mystery of these bones was solved. Until then the scholars of the 17th century speculated about the origin of these enigmatic bones. Olearius (1674), who illustrated a second specimen, mentioned the possibility that these bones could have grown Figure 19. Chaetodipterus faber, NMW 93686, SL 165 mm; skeleton with hyperostotic bones (from Smith-Vaniz et al. 2024, modified). Abbreviations: APT: first pterygiophore of the anal fin, CL: cleithrum, HS: haemal spines, PT: first and second pterygiophore of the first dorsal fin, R: ribs, SOC: supraoccipital. Scale bar: 50 mm. Photo by K. Schiller. anhmw.pensoft.net Harald Ahnelt et al.: Fish skeletons with hyperostosis in NHMW22 in the ground, a viewpoint that probably goes back to fossil finds of bones for which there was no other explanation at the time. A third specimen was mentioned by Jacobaeus (1696) also found no explanation for its origin. The first to realize the origin of this bone as a bone of a fish was the surgeon William Bell (1793) who illustrated a specimen with numerous hyperostotic bones (Fig. 3). He also realized that these hyperossifications were not the result of a disease. Initially believing that he found a fish with exostoses, Bell detected that the same structures also occur in other specimens. Therefore, he concluded “them to be natural”. Bell mentioned the spongy structure Figure 20. Hyperostotic bones of different consistency. A, B. Spongy; C–G. Compact. A. “Os wormianum” (first anal-fin pterygiophore of Chaetodipterus faber) in longitudinal view (NMW 97939); B. Rib of Caranx hippos (NMW 93700) in longitudinal view; C–G. Zeus faber; C. Supracleithrum in longitudinal view (NMW 91917); D, E. Buckler scale in longitudinal (D) and in transversal view (E) (NMW 60029); F. Dorsal-fin spine in longitudinal view (NMW 92285); G. Base of dorsal spine with lateral wing-like extensions in transversal view (NMW 92285). MicroCT images V. Winkler. Scale bars: 10 mm (A, B); 5 mm (C–G). MicroCT images by V. Winkler. Figure 21. Hyperostotic bones, of which MicroCts are shown in Fig. 20. A. “Os worminaum” (first anal-fin pterygiophore) (NMW 97939) in lateral view, 101.4 mm; B. Rib of Caranx hippos, (NMW 93700) in lateral view, 73.5 mm; C–F. Zeus faber; C. Supracleithrum in dorsal view, 45.3 mm, (NMW 91917); D. Buckler scale in ventro-lateral view (above) and in dorsal view (below), 16.3 mm (NMW 60029); E. Dorsal-fin spine in postero-lateral view, 51.7 mm, tip broken (NMW 92285). Two artificial holes in C for mounting on a plate. Photos by H. Ahnelt. Annals of the Natural History Museum Vienna 126 2025, 5–25 anhmw.pensoft.net 23 of these affected bones and that they are “so soft to be easily cut with a knife.” Also, Klein (1948) characterized the hyperostotic first pterygiophore of the anal fin of Chaetodipterus faber as firm foam surrounded by walls of laminar bone but described this bone as a hard structure. Possibly these differences in the hardness of the bone can be explained by the fact that Bell (1793) examined fresh specimens, whereas Klein (1948) examined old and dry museum material. In his famous work “De l’Hyperostose chez l’Homme et chez les Animaux” Gervais (1875) illustrated not only hyperostotic fish bones but also vertical and transversal sections through the hyperostotic part of an anal-fin pterygiophore. In these figures, the spongy structure of the hyperostotic material is very well presented. Subsequently, this spongy structure of hyperostotic bone has been described for a series of fish species (e.g., Grabda 1982; Schlüter et al. 1992; Meunier and Desse 1994; Meunier et al. 2010). We found this spongy condition of the affected bones also in our specimens except for Z. faber. Two types of hyperostosis Generally, hyperostosis refers to a pathological or non-pathological overgrowth of bone. Such overgrowth of bone has been described for fishes (Smith-Vaniz et al. 1995), amphibians (Clemente-Carvalho et al. 2009), reptiles (McWilliams and Leeson 2001), birds (Sakas 2002) and mammals (Kraneburg et al. 2014) including men (Walker et al. 2009). However, the vertebrates in which hyperostosis most frequently occur are fishes, more precisely, teleost fishes (Smith-Vaniz et al. 1995; Meunier et al. 2008; Witten and Huysseune 2010). Teleost fishes can be subdivided by the nature of their bone in (1) basal teleosts which are characterized by bone with osteocytes (cellular) and (2) advanced teleosts which are characterized by bone without osteocytes (acellular) (Hall and Witten 2018; Davesne et al. 2019; for exception see Davesne et al. 2019). The vast majority of fishes with hyperostosis are advanced teleost fishes, generally Acanthopterygii (Smith-Vaniz et al. 1995; Davesne et al. 2019). So far described, hyperostotic bone of acanthopterygian fishes is typically spongious surrounded by a thin sheet of cortical bone (e.g., Grabda 1982; Schlüter et al. 1992; Meunier and Desse 1994; Meunier et al. 2010). Most of these spongious hyperostoses are externally not visible and do not affect the body shape of the fishes, especially those of the postcranial skeleton. Exceptions are cranial hyperostotic bones like the frontal or the supraoccipital in e.g., Haemulidae (Meunier and Desse 1994) or Ephippiidae (Jawad 2013, this study) which give these skulls a greater appearance. This is different in the second type of hyperostosis where the bone is compact and laminar. This type occurs in Zeus faber. The first to study this bony structure were Meunier and Béarez (2019) who, described the histology of the bony scutes of this fish. These authors characterized the bony tissue as pseudolamellar bone with annual wide (spring, summer) and narrow (autumn, winter) growth marks which correspond to body growth during the year (Fig. 20D, E). Seemingly the same structure of bone is visible in the other hyperostotic bones of Zeus faber, which are of similar massive appearance, especially the supracleithrum (Fig. 20C, G). Contrary to the bones affected by spongy hyperostosis the hyperostotic bones of Zeus faber are all externally visible. In contrast to the typical described hyperostotic bones of which the functional basis is not known (Tiffany et al. 1980; Meunier and Huysseune 1992; Smith-Vaniz et al. 1995), those of Zeus faber seem to have at least a protective function (Fig. 14B). What both types of hyperostosis have in common is the growth with age (Smith-Vaniz et al. 1995; Meunier and Béarez 2019). Onset of hyperostosis Hyperostosis is documented only in large, adult specimens of a species and is usually not apparent in juveniles (Klein 1948; Murty 1967; Bortone 1977; Tiffany et al. 1980; Smith-Vaniz et al. 2024) (Fig. 17A). What is known from the few non-adult specimens available for this study, which show onset of hyperostosis is, that the timing of this overgrowth of bone is seemingly not simultaneous but sequential (Figs 17, 18). Less distinct in Paralonchurus peruanus (Fig. 17), this is distinctly recognizable in two species of Drepane. In adults of this species (Fig. 18A), the frontal and supraoccipital as well as the first dorsal-fin pterygiophore and the first anal-fin pterygiophore are hyperostotic. In juveniles of Drepane punctata of a size of 145 mm (Fig. 18B) and of Drepane longimana of a size of 150 mm SL (radiograph, not shown) just the frontal had distinct signs of hyperossification. The pterygiophores of the fin, heavily hyperossified in adults, are at this size still not affected. Interestingly, the hyperostotic part of the frontal appears in a different colour than the rest of the bones of the head, it is white (Fig. 7B). The other bones, including the still non-hyperostotic part of the frontal, are much darker. Possibly this difference in color is caused by the remodeling of acellular (anosteocytic) bone tissue to cellular (osteocytic) bone tissue as hyperostotic bone of acanthopterygian fishes shows spatial juxtaposition of these two bone tissues (Parenti 1986; Smith-Vaniz et al. 1995). In Paralonchurus peruanus, the onset of hyperostosis is apparently around a size of 230 mm SL. Specimens with a size of 320 mm SL were already fully affected by this phenomenon (Fig. 17). Two further rare exceptions concerning our knowledge on the onset of hyperostosis in fishes are Lepidopus caudatus (Euphrasen 1788) and Selene setapinnis (Mitchill 1815). In Lepidopus caudatus, the first signs of hyperostosis appeared at a size of 930 mm SL and was fully developed in specimens with a size of 1010 mm SL (Giarratana et al. 2012). In Selene setapinnis, hyperostosis occurred in specimens with a size of 236 mm TL. anhmw.pensoft.net Harald Ahnelt et al.: Fish skeletons with hyperostosis in NHMW24 This excessive overgrowth of bone was then found in all specimens >260 mm TL (Tuna et al. 2023). In order to obtain a clear picture of the onset and the ontogenetic development of hyperostosis in different fish species, further series of different size classes are required. Acknowledgements We thank Anja Palandačić (NHMW), who made the fish skeletons of the Ichthyological Collection of the NHMW available to us, as well as Nina Bogutskaya and Katharina Schiller (NHMW) for X-ray images and photos of fish skeletons. We also thank Martin Krenn (NHMW) for information about the painting which shows Franz Steindachner. We are grateful to Timo Moritz (Leibnitz-Institute for the Analysis of Biodiversity Change, Hamburg) and Werner Schwarzhans (University of Copenhagen) for their thorough reviews that improved the manuscript. References Abel O (1912) Grundzüge der Palaeobiologie der Wirbeltiere. E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, 708 pp. https://doi. org/10.5962/bhl.title.61833 Aguilera O, Rocha I, Lopes MS, Lima I, Lopes RT, Machado AS, Guimarães RB, Crapez MAC, Tenório MC, Nepomuceno A (2017) The bone degenerative processes in senile fishes from Holocene Brazilian shell mounds. 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